Wind energy is a vast resource that can be tapped in a distributed manner. Economically viable wind energy technology depends on having good power extraction efficiency in all subsystems of the wind energy conversion system (WECS).
The power conversion stage is an important source of efficiency losses in a WECS and also contributes significantly to system size and cost. A reasonable trade off between efficiency, costs and size can be achieved by using a matrix converter. The matrix converter is a forced commutated converter, which uses an array of controlled bidirectional switches as the main power elements to create a variable output voltage with unrestricted frequency. It does not have a dc-link circuit, and no large energy storage elements.
Methods for converting alternating current (AC) power from one frequency to another can be broadly classified into (i) indirect methods and (ii) direct methods.
Indirect methods include two or more stages of conversion and an intermediary direct current (DC) link. For example, a diode rectifier can be used to convert AC power to a DC power. Alternatively, a pulse width modulation (PWM)-controlled rectifier can be used to enable bidirectional power flow. The rectification is realized in conjunction with a DC-link capacitor. A voltage source inverter converts DC back to AC with variable frequency and amplitude.
Direct methods involve an array of static power switches connected directly between the input and output terminals. The basic operating principle is to piece together an output voltage waveform with the desired fundamental component from selected segments of the input voltage waveforms. The most common form of direct method-based converters are the cycloconverters. In general there are two types of cycloconverters: (i) naturally commutated and (ii) forced commutated. Naturally commutated cycloconverters use thyristors that are switched naturally by voltage levels of the AC supply. A forced commutated cycloconverter, such as the matrix converter, uses switches that operate independently of the source and load voltages. They require auxiliary commutating circuits.
A matrix converter interfaced with a variable-speed wind turbine incorporating a doubly-fed induction generator is described in U.S. Pat. No. 6,566,764 issued May 20, 2003 to Rebsdorf and Halle. A protection circuit is used for protecting the circuit from over voltages, and retains control of the matrix converter after grid disruption. This system only applies to induction generators that are not suitable for all wind power applications. In addition it does not integrate rotor and generator control. Further it is not designed for permanent magnet generators that are more efficient and particularly well-suited for small wind turbines.
A three-phase matrix converter for converting AC voltages of predetermined amplitude and frequency into AC voltages of any amplitude and frequency and a method for operating the same, using bidirectional switches is described in U.S. Pat. No. 5,949,672 issued Sep. 7, 1999 to Bernet.
U.S. Pat. No. 6,826,065 issued Nov. 30, 2004 to Cheket, et al. describes a method of commutation of current by bi-directional switches for matrix converters with at least three input phases. A matrix converter that allows for completely natural commutation between phases is presented in U.S. Pat. No. 6,519,170 issued Feb. 11, 2003 to Lacaze et al. Alternative commutation methods are described in U.S. Pat. No. 5,594,636 issued Jan. 14, 1997 to Schauder and U.S. Pat. No. 5,949,672 issued Sep. 7, 1999 to Bernet. Short circuit currents through the switches and overvoltages on circuit elements are important concerns in a matrix converter system. U.S. Pat. No. 6,603,647 issued Aug. 5, 2003 to Briesen, et al. describes a method for controlling freewheeling paths in a matrix converter. U.S. Pat. No. 6,496,343 issued Dec. 17, 2002 to Mahlein, et al. presents an overvoltage protection apparatus for a matrix converter.
The preceding seven patents only addressed control of the matrix converter and not the integrated control of the converter with other elements in a system. Thus, there is a need for a WECS using a matrix converter which integrates control of the converter with other elements of the WECS.